Pomegranates inspire new battery design

A structural breakthrough could pave the way for a shift from carbon to silicon that would multiply energy storage 10 times. By Philip Dooley.

Pomegranate seeds, encased in their skin, were the inspiration for a new battery design where nanoscale 'seeds' of silicon are housed in an outer carbon shell. – iStock

It’s amazing what you can do with a pomegranate. Humans have used the fruit for thousands of years as a tonic for the heart, a cure for diarrhoea, and a female contraceptive. And now, in research published in Nature Nanotechnology this February, for electricity storage.

This time, it is not the fruit itself being used but its structure. It has inspired an ingenious new design for lithium batteries that could increase their capacity many times over.

Yi Cui and his colleagues from Stanford University have used the design to replace the standard battery’s carbon anode – the sponge that soaks up lithium ions as the battery is charged – with one made of silicon. Silicon can store 10 times the amount of lithium that carbon can. In other words, it theoretically stores 10 times the energy. However, silicon has a fundamental problem as an anode material: it cracks and falls apart after only a few charging cycles.

The cracking occurs because silicon soaks up so much lithium during charging that it swells dramatically, tripling in volume. As it repeatedly swells and shrinks as the battery is used, the anode disintegrates.

Microscopic clusters form a fine black powder that can be coated on foil to create an anode. Middle: A single cluster. Right: In this close-up of a cluster, a silicon nanoparticle can be seen inside its 'skin', with space to swell during battery charging. – Nian Liu, Zhenda Lu and Yi Cui/Stanford University

Cui’s bright idea is to not use a solid slab, but nanoscale “seeds” of silicon – tens to hundreds of thousands of them – loosely encased like pomegranate seeds each in an outer carbon skin. The skin leaves plenty of room for the silicon to shrink and grow in the charging cycle, and the particles’ tiny size and spherical shape means they don’t fracture. Lithium ions are very small and can easily pass through the thin carbon layer of the skin, so the design exploits silicon’s storage capacity while retaining carbon’s rigidity and conductivity.

The rigid outer skin also keeps the silicon separate from the battery’s electrolyte, avoiding power-sapping side-reactions between them. These reactions have plagued other recent efforts to improve the lithium battery.

It was while wrestling with the electrolyte problem in 2012 that Cui first came up with the pomegranate idea, he says. The challenge then was to develop a process to manufacture the “pomegranate seeds”. He explains: “The breakthrough is both the design and coming up with a method to produce them.”

A model of the pomegranate-inspired design before and after electrochemical cycling. The space around each silicon nanoparticle allows it to expand without either deforming the overall shape, or cracking the particles themselves. The carbon framework ‘skin’, meanwhile, prevents the silicon particles coming into contact with the electrolyte. – Nian Liu, Zhenda Lu and Yi Cui/Stanford University

The team’s trick was to first coat the silicon seeds with a thick layer of silicon dioxide, before adding the thin carbon skin. Washing with acid then stripped away the silicon dioxide through tiny pores in the carbon, leaving the pure silicon nanoparticle rattling around inside its protective carbon cage. Finally, clusters of encapsulated nano particles are wrapped within the thick outer carbon coating. “With some tweaking, scaling up the process should be straightforward,” Cui says.

That’s when we’ll really know how the pomegranate battery performs, says Philip Daries, director of New South Wales industrial battery specialists Regal Electro. “We are seeing an avalanche of new battery technologies at the moment. I’m reading about a new one every week. The proof of the pudding will come when it is used in manufacture, a year or two down the track.”